The present invention relates to immersion microscope objectives. More particularly, the present invention relates to an apochromat-class immersion microscope objective having a magnification of about 60 and a numerical aperture (NA) of 1.4.
Conventional objectives which are similar to the immersion microscope objective of the present invention, which will be described later, include those disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 59-155822 and 61-240218. Japanese Patent Application Laid-Open (KOKAI) No. 59-155822discloses an apochromatic objective having a favorably flattened image field, which includes, in order from the object side, a 1-st lens unit having a buried lens, a 2-nd lens unit having a meniscus positive lens, a 3-rd lens unit having a cemented lens composed of three lens elements, that is, positive, negative and positive lenses, a 4-th lens unit having a cemented lens composed of three lens elements, that is, negative, positive and negative lenses, a 5-th lens unit including a cemented meniscus lens having a strong concave surface directed toward the image side, and a 6th lens unit including a cemented meniscus lens having a strong concave surface directed toward the object side. Japanese Patent Application Laid-Open (KOKAI) No. 61-240218discloses an apochromatic objective having a favorably flattened image field, which includes, in order from the object side, a 1-st lens unit having a buried lens, a 2-nd lens unit having a meniscus positive lens, 3-rd and 4-th lens units each including a cemented lens composed of two lens elements, that is, negative and positive lenses, a 5-th lens unit having a cemented lens composed of three lens elements, that is, negative, positive and negative lenses, a 6th lens unit including a cemented meniscus lens having a strong concave surface directed toward the image side, a 7-th lens unit including a cemented meniscus lens having a strong concave surface directed toward the object side, and an 8-th lens unit including a meniscus positive lens having a concave surface directed toward the object side. The above-described two conventional objectives have something in common with each other in their arrangements: the 1-st lens unit has a buried lens; a cemented lens composed of three lens elements, that is, negative, positive and negative lenses, is used; and a Gauss type lens, which has concave surfaces facing each other, is disposed in the rear part of the objective.
However, the objective disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 59-155822 is not satisfactorily corrected for spherical and axial chromatic aberrations. On the other hand, the objective disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 61-240218 is satisfactorily corrected for axial aberrations but requires a large number of constituent lens elements, i.e., 15 lens elements arranged in 8 lens units.